Axial flow fan having fully streamlining flexible blades

ABSTRACT

An axial flow fan for use on variable speed engines for pulling a cooling fluid past a cooling coil, particularly at low speeds, responsive to rotation of a rotary part driven by the engine characterized by a spider having an axis of rotation and a plurality of radially extending arms and adapted for being mounted on and rotated with the rotary part; a plurality of fan blades that are secured, respectively, to the arms along a line of attachment and have; in addition to the leading edge portion, attachment portion and flexible, trailing, flow-inducing portion; a hinge portion intermediate the attachment portion and the flowinducing portion. The hinge portion is further characterized by a flexural groove traversing the major portion, and preferably all, of the length of the blade, parallel with the attachment portion and located transversely and trailingly behind the arm for allowing the flow-inducing portion to go into its fully streamlined position without contact with the arm for full streamlining and low power consumption. Also disclosed are specific constructions, including critical dimensions, materials of construction and the like for effecting optimum results.

United States Patent [1 1 Arrington et al.

[ Oct. 21, 1975 AXIAL FLOW FAN HAVING FULLY STREAMLINING FLEXIBLE BLADES [75] Inventors: Earl J. Arrington, Arlington; Jerry E. Deal, Hurst; Roger C. Hunsaker, Fort Worth, all of Tex.

[73] Assignee: Fort Worth Pressed Steel Corporation, Fort Worth, Tex.

22 Filed: June 6,1973

21 Appl. No.: 367,583

[52] US. Cl 416/132; 416/240 [51] Int. Cl? F04D 29/38 [58] Field of Search 416/132, 132 A, 240

[56] References Cited v UNITED STATES PATENTS 2,072,196 3/1937 Berger 416/132 3,490,686 1/1970 Weir 416/132 X 3,664,165 5/1972 Harvill et a1. 416/132 UX 3,698,835 10/1972 Kelly 416/132 3,759,630 9/1973 Freeman et al 416/132 D187,272 2/1960 Twist 416/132 A UX Dl98,242 5/1964 Twist 416/132 A UX FOREIGN PATENTS OR APPLICATIONS 559,674 9/1932 Germany 416/240 Primary ExaminerEverette A. Powell, Jr. Attorney, Agent, or Firm-Wm. T. Wofford [57] ABSTRACT An axial flow fan for use on variable speed engines for pulling a cooling fluid past a cooling coil, particularly at low speeds, responsive to rotation of a rotary part driven by the engine characterized by a spider having an axis of rotation and a plurality of radially extending arms and adapted for being mounted on and rotated with the rotary part; a plurality of fan blades that are secured, respectively, to the arms along a line of attachment and have; in addition to the leading edge portion, attachment portion and flexible, trailing, flow-inducing portion; a hinge portion intermediate the attachment portion and the flow-inducing portion. The hinge portion is further characterized by a flexural groove traversing the major portion, and preferably all, of the length of the blade, parallel with the attachment portion and located transversely and trailingly behind the arm for allowing the flow-inducing portion to go into its fully streamlined position without contact with the arm for full streamlining and low power consumption. Also disclosed are specific constructions, including critical dimensions, materials of construction and the like for effecting optimum results.

21 Claims, 8 Drawing Figures AXIAL FLOW FAN HAVING FULLY STREAMLINING FLEXIBLE BLADES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to variable speed fans for moving a cooling fluid to cool a coil that moves relative to the cooling fluid. More particularly, it relates to axial flow fans of the so-called flexible, type for use on variable speed engines for pulling a cooling fluid, such as air, past a cooling coil, such as a radiator, particularly at low speeds, responsive to rotation of a rotary part driven by the engine; the fan being operable to reduce the parasitic load and noise level on the engine at high speeds when the air flows past the radiator of its own volition.

2. Description of the Prior Art In the early stages of automotive development, the construction of the fans was very simple because high speeds were not attained and fan design was merely one of the minor considerations in the overall design of a horseless carriage. With the advent of the high speed automobiles and high speed automobile engines, however, the need for reducing the parasitic power consumption and noise by the fan became apparent, since the speed was adequate to effect most of the desired cooling. The initial approach in this direction was that of decoupling the fan from the engine. The decoupling was effected by a variety of devices; such as electric or magnetic clutches operated in response to a thermally responsive element, and fluidic clutches operated responsive to temperature effects on the rheological properties of coupling fluids. Typical of the patents relating to this area are U.S. Pat. Nos. Re 25,481; 3,191,733; and 3,490,686. Another approach that was employed to effect a reduced power consumption and lower noise by the fan was the use of flexible blades on the fan, along or in conjunction with a decoupling device. Typical patents pertinent to the flexible blade structure are US. Pat. No. 2,032,224; 3,044,557; 3,289,924; 3,406,760; and 3,698,835; as well as the above noted 3,490,686. Of the patents, the latter two are deemed most pertinent. In US. Pat. No. 3,490,686, longitudinal ribs 30a are employed to provide lateral stiffness without interfering with the blades ability to flex transversely. In U.S Pat. No. 3,698,835, a tapered arcuate hinge bend is provided with both edges touching a pitched spider arm in order to allow flexure of the blade, yet still provide stiffness, support and pitch, or angle of attack. Even the latter highly advanced and sophisticated blade, however, has been found not totally satisfactory in that it still has too high a power consumption and noise level at high speeds for reasons which will become more clearly apparent from the descriptive matter relating to this invention hereinafter.

Moreover, we have found that all of the available flexible fans have considerable undesirable tip dance", or erratic movement of the blade tips at rates of rotation above about 2,000 revolutions per minute (rpm). The actual point of flex of the blades was not very well controlled, the total amount of flex being centered around the radially outermost point of the blade contact with the spider arm, even where additional stiffeners were employed. We tried building a fan with stainless steel blades folded over the leading edge to eliminate the need for an additional stiffener. This blade, similarly as with the prior art blades, had too much tip dance, as well as other problems. The tip dance can be examined by stroboscopic, or strobe, light to check the position of the tips of the fan blades with respect to a fixed point as the tips of the fan blades rotate past the point.

Accordingly, it is an object of this invention to provide an axial flow fan having flexible fan blades that obviates the disadvantages of the prior art and provides an axial flow flexible fan that has the lowest power consumption, the least noise, and has the least tip dance of any fan yet developed.

It is also an object of this invention to provide an improved axial flow flexible fan construction that provides the above object and is durable, maintenance free and dependable.

These and other objects will become apparent from the descriptive matter hereinafter, particularly when taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a phantom internal combustion engine employing a flexible fan construction in accordance with an embodiment of this invention.

FIG. 2 is a partial end view showing a fan blade of the embodiment of FIG. 1 and its spider arm.

FIG. 3 is a plan view of the blade in flat before it is formed into final shape.

FIG. 4 is a plan view of the fan blade of FIG. 3 after it has been formed into shape.

FIG. 5A is a partial side elevational view, taken from the rear along the line VA-VA of FIG. 5B.

FIG. 5B is a partial plan view of the embodiment of FIG. 1.

FIG. 6 is a partial cross sectional view of another embodiment of this invention.

FIG. 7 is a partial cross sectional view of still another embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, an axial flow, flexible blade fan 11 is illustrated connected with a rotary part (not shown) driven by the engine 15. Conventionally, the rotary part includes a shaft having a pilot 13 in its outer end with plurality of bolts 28 in apertures 27 to retain the fan in place and centered. The rotary part and shaft are driven by the engine 15 by way of suitable fan belt 17, in accordance with conventional practice. The fan 11 is constructed in accordance with this invention. It includes a spider 19 and a plurality of flexible fan blades 21 that are connected with respective arms of the spider 19 and extend radially outwardly.

As indicated, the spider 19 is adapted for being mounted on the rotary part. Ordinarily, the spider 19 has an axis of rotation that coincides with the central longitudinal axis of the shaft of the rotary part. The spider 19 has a plurality of radially outwardly extending arms 23. As illustrated, all of the arms 23 are co-planar and their common plane is substantially perpendicular to the central longitudinal axis of the shaft, or the axis of rotation of the spider 19. In fact, the spider 19 comprises two planar halves that match to facilitate emplacement on both sides of each respective fan blade 21. If desired for greater air flow, the respective arms 23 may have an angle of attack with respect to the air stream and thus would not be co-planar.

I Thespider 19 has a hub section 25 that is adapted to be mounted on the rotary part by any conventional means. As illustrated, the means for mounting comprises a plurality of four elongate apertures 27, FIG.

58, adapted to receive bolts 28, FIG. 1, on the rotary part. The hub section 25 also has, a central aperture 29 for receiving the pilot 13. The pilot 13 centers the fan, and thebolts 28 in the elongate apertures 27 retain the fan 1 1 on the rotary part for ensuring rotation as a unit.

The plurality of fan blades 21 are transversely elastically flexible and are constructed and mounted in such a way that upon rotation of the fan 11 in the direction of the arrow 30, FIG. 1, air will be pumped in a direction parallel to ,the axis of rotation of the fan 11; that is, inwardly through the radiator (not shown) and toward the rear of the engine 15. To accomplish this movement of air, each fan blade has a rearwardly curved trailing section that tends to flatten out at increasing pressure differential.

It is believed that a discussion of the operation of the flexible fan will contribute to an understanding to the nuances and details of construction, some of which are subtle and appear insignificant without having understood the operation. Accordingly, the following material describes the operation of the flexible fan 11. As rotation starts, each fan blade 21 effects a flow of air. Eachof the trailing sections are subjected to pressure equal and opposite the force parallel to the flow of air. Expressed otherwise, the reaction pressure and force is in the longitudinally forward direction, tending to straighten out, or flatten, the curved trailing sections of the blades. As the blades flatten at increasing air speeds, they use less horsepower than a fixed blade,

I thereby increasing the efficency of the operation of the the engine slows down, the reverse procedure occurs and the fan blade trailing section again becomes curved because of the elasticity of the fan blade. As will be appreciated, this is repeated many times in the life of the automobile. Repeated flattening or flexing of the blades introduces fatigue stresses into the respective blades and their connections with the spider arms 19, in the'absense of this invention, that may lead to early failure. Consequently, proper design of the fan requires -very sophisticated engineering and careful attention to seemingly small details to avoid the crystallization and fatigue failure that has been commonin the prior art fan blades.

The plurality of fan blades 21 are secured, respectively, to the spider arms 23 along respective lines of attachment 31, FIG; 3.

Each fan blade 21, per se, includes a leading edge portion 33, FIGS. 3 and 4; an attachment portion 35; a trailing section, or flow-inducing portion 37; and a hinge portion 39, FIG. 4.

The leading edge portion 33 is planar and has substantially the same thickness as the remainder of the blade 21. It is disposed forwardly of the attachment line 31, and includes a leading edge 41. The line of its leading edge 41 intersects a line defined by a-planar projection of the curved endv 43 at a point 45, FIGS. 3 and 4,

that is substantially aligned with the attachment line 31 U such that tip dance of high speed is maintained at substantially zero. The reason for this is not exactly clear. The following theory is given by way of explanation only, and not by way of limitation. It is theorized that as the curved, trailing, flow-inducing portion 37, FIG. 5B, tends to straighten out, there is, in addition to the main flexure of the hinge portion 39, a slight flexure about the line of attachment 31 where the fan blade is free of its respective spider arm 23; for example, the

leading edge portion 33 radially exteriorly of its spider arm. Because of this flexure, it is theorized, there will be a portion of the leading edge portion that will be subjected to the airstream, as the fan 11 is rotated to generate a force coming out of the paper in FIG. 58, illustrated by the tip of an arrow 47. This force at least partially counters the pressure generated on the fan blade to force it into the paper, illustrated by the crossed feathers of the arrow 49. With this compensating force, there is sufficient inherent strength in the fan blade 21 to resist the tendency of the top to be moved responsive to the reaction force exerted by the air on the fan blade. The respective imaginary arrows 47 and 49 are illustrated in FIG. 5A to assist in understanding I i the theory. The arrow 49 is illustrated as being applied at some arbitrary point analogous to a center of pressure. Moreover, the inert mass exteriorly of the arms23 is reduced. If the mass were retained it would contribute to potential instability and tip dance without contributing to increased air flow. Regardless of the reasons for. the efficacy, it is an observable phenomena that tip dance of the fan blade 21 can be held at sub stantially zero by having, in conjunction with the hinge. portion 39, the leading edge portion extending forwardly of the line of attachment 31, as described and 1 illustrated.

The attachment portion 35 overlaps a portion of the spider arm 23 and is affixed thereto so as to rotate in unison therewith about the axis of rotation of the fan 11. The attachment portion 35 is planar and coengages the planar portion of the spider arm 23. In fact, as illustrated, the spider arm 23 comprises two portions 23A and 238, FIG. 2, with the attachment portion 35 of the fan blade sandwiched therebetween. As illus trated, rivets 51, FIGS. 2 and 5A and 5B, are employed for fastening the two halves of the respective spider arms 23 together. The line of attachment 31 is parallel with the trailing edge of the spider arms 23. The arms 23 have a leading edge extending forwardly of the attachment line 31 for bracing the radially innermost.

portion ofthe leading edge portion 33 of the blade 21.

The flow-inducing portion 37 has a generally arcuate, angularly disposed shape trailing and extending rearwardly of the respective spider arms 23 and the attachment portion '35 for efficientlyeffecting a flow of air at low speed of relative movement between the air and the fan 11. The trailing, flow-inducing portion 37 has.

sufficient flexure to streamline into position behind its rotating spider arm 23 at high relative speeds between the air and the fan for low power consumption. As illustrated, the flow-inducing portion 37 is formed by bending a stainless steel fan blade about a cylinder having a radius of about 0.75 inch and releasing it to allow it to flex back into its correct shape. This degree of arcuateness may be varied depending upon the applications. For example, a fan blade that is satisfactory for a high speed atuomobile engine may. not be satisfactory for an engine for a larger vehicle, such as a truck, or off the road type equipment. The design of the trailing edge portion 37 is within the skill of the art, however; is not being claimed, per se, herein; and need not be described in detail to lengthen this already lengthy specification.

The hinge portion 39 is the point of major departure of this invention from the prior art. ltis located intermediate the attachment portion 35 and the flowinducing portion 37. Importantly in comparison with the prior art, it is located behind the trailing edge of its respective spider arm 23. The hinge portion 39 includes a flexural groove 53, FIGS. 2 and 4, traversing longitudinally of the blade 21 and parallel with the attachment portion 35; and a tangentially curved region 55 adjacent the flexural groove 53 and leading into the attachment portion 35. As illustrated, each respective blade 21, has its flexural groove 53 extending rearwardly with respect to the front 57, FIG. 2, of the fan blade 21 and spider arm 23A; and has a second curved region 61 for connecting smoothly with the flowinducing portion 37.

The flexural groove 53 extends at least the major portion of the length of the blade 21. Preferably, and as illustrated, the flexural groove 53 extends the full length of the blade 21. The flexural groove 53 traverses parallel with the attachment portion 35 and is located transversely and trailingly behind the spider arm 23 for allowing the flow-inducing portion 37 to go in its fully streamlined position without contact of any part of the hinge portion 39 or the flow-inducing portion 37 with the spider arm 23. This structure is advantageous over the prior art structure in that it allows full streamlining of the fan blade behind the spider arm 23 for low power consumption; and it prevents wear on the fan blade 21 at a line of contact between the fan blade and the spider arm 23 as in the prior art. The flexural groove 53 has at least one arcuate region that has a cross sectional radius of curvature that is within a critical range at all points. Expressed otherwise, the radius of curvature of the flexural groove 53 along any longitudinally extending line, or axis, contained therewithin is within the critical range. The radius of curvature is large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough not to protrude beyond the thickness, or dimensions, of the spider arm 23 when the flow-inducing portion 37 is streamlined at high speed into its fully streamlined position for low power consumption. The radius of curvature of the arcuate region along any given line is at least 0.2 inch to prevent the line of stress sufficient to cause crystallization and is no more than 0.3 inch to be small enough not to protrude beyond the dimensions of the arm when the flow-inducing portion 37 is streamlined at high speed. As illustrated, the arcuate region of the flexural groove 53 is a portion of a circular cylinder, or has a cross sectional shape that is substantially a portion of an arc of a circle, with a radius of curvature R of about 0.25 inch i 0.05 inch; and extends rearwardly with respect to the attachment portion 35. Specifically, the arcuate region of the flexural groove 53 is formed by conforming development around a cylinder having a radius of about 0.24 inch.

The central longitudinal axis of the arcuate region of the flexural groove 53 is located about 0.75, i 0.20 of an inch transversely trailing the attachment line 31 of the fan blade and, as indicated hereinbefore, transversely trailing the rear edge of the spider arm 23 for the delineated advantages.

The tangentially curved region 55 is connected with the flexural groove 53 and with the attachment portion 35, leading smoothly into the attachment portion. The

tangentially curved region 55 has a radius of curvature at all lines contained therewithin large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough to become tangent to the plane of the attachment portion 35 without having the flexural groove protrude past the plane of the outermost part of the spider arm 23. The tangentially curved region 55 has a radius of curvature along any given longitudinal line contained therewithin that is at least 0.10 inch to prevent the line of stress sufficient to cause crystallization and is no more than 0.20 inch to become tangent to the plane of the attachment portion. As illustrated, the tangentially curved region is substantially a part of a circular cylinder having a radius of curvature R of about 5/32 inch t 0.050 inch.

The second curved region 61 is disposed transversely adjacent and trailing the arcuate region of the flexural groove 53. The second curved region 61 is connected smoothly with the arcuate region of the flexural groove 53; and is connected smoothly with and leads into the flow-inducing portion 37 The second curved region 61 has a radius of curvature along any line contained therewithin that is large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough to lead into the flow-inducing portion 37 without protruding beyond the plane of the forwardmost part of the spider arm 23 and into the airstream, thereby allowing low power consumption at the fully streamlined position of the flow inducing portion 37 The second curved region 61 has a radius of curvature along any given longitudinal line contained there within that is at least 0.10 inch to prevent the line of stress sufficient to cause crystallization and is no more than 0.20 inch and is sufficient to have the forwardmost portion of the flow-inducing portion rearward of the forwardmost portion of the spider arm 23 by a distance equal to the thickness of the flow-inducing portion 37 such that it does not protrude forwardly of the plane of the spider arm even when the flow-inducing portion 37 is forced into its fully streamlined position. As illustrated, the second curved region 61 is substantially a part of a circular cylinder and has a radius of curvature of about 5/32 inch t 0.050 inch.

Preferably, the tangentially curved region 55, the flexural groove 53 and the second curved region 61 are so dimensioned and connected that the forwardmost part of the flow-inducing portion 37 is offset from the front plane of the attachment portion 35 a distance D, FIG. 2, that is at least as great as the thickness of the blade and no more than 0.1 inch to allow full flexure into the fully streamlined position for best reduction in turbulence and noise at high rotational speeds.

We have built a 15 inch diameter, six blade standard flex fan employing the criteria delineated hereinbefore. It had a maximum static unbalance of less than 0.25 ounce inch. The spider material employed was 14 gauge steel with a black enamel finish. Rivets 7/23 inch in diameter were employed in effecting a connection of the fan blades intermediate the two halves of the spider arms 23. Larger sizes of fans have been manufactured and the criteria described hereinbefore has been found to be satisfactory. Employing the construction delineated hereinbefore, with stroboscopic light and tachometer, the blades were examined at rotational speeds of from 1,000 to 5,000 rpm with no visible dance at the tips.

As implied hereinbefore, the flexural groove 53 need not extend rearwardly with respect to the spider arms 23, but can extend forwardly as illustrated in FIGS. 6 and 7. In the embodiments illustrated in FIGS. 6 and 7, the flexural groove 53 extends forwardly with respect to the forward side 57 of the spider arms 23.

As illustrated in FIG. 6, only the tangentially curved region 55 and the flexural groove 53 are employed, since the flexural groove 53 blends smoothly into the flow-inducing portion 37. This has been referred to as the stepped fan construction. Otherwise, the construction is the same as delineated hereinbefore.

In the embodiment of FIG. 7, the tangentially curved region 55, the flexural groove 53 and the second curved region 61 are employed. Similarly as with respect to the embodiment of FIG. 6, the construction and attachment of the flow-inducing portion of the fan blade 21 is the same as described hereinbefore.

There are some general considerations that merit noting.

The spider arms 23 may be made of any suitable material. Preferably, they are formed of lightweight metallic material, such as steel, aluminum, magnesium or the like. In some instances, it is possible to employ plastic, although plastic material ordinarily lacks the strength and durability of the metallic components. Suitable plastics may comprise thermoplastic materials, such as the acrylonitrile butadiene styrene copolymer (ABS); with or without additional materials, such as fiberglass filaments, for strength.

In like manner, the fan blades may be formed of any suitable elastic metal or plastic materials. We have found it preferable to employ stainless steel in making the fan blades, since the stainless steel has the desired characteristic of elasticity and durability to be readily formed by processes exceeding the elastic limit of the material, yet retain full elasticity when installed as the fan blade. Flexible plastic fan blades have been employed in the past, but they ordinarily require heat during the forming process and do not retain all of the clasticity necessary to prevent fatigue failure upon prolonged use.

While the attachment of the fan blades 21 to spider arms 23 has been described hereinbefore by way of sandwiching of the attachment portion 35 between two halves of the spider arm 23, employing rivets or the like, any other suitable method may be employed as long as the requisite durability and trouble-free opera-' tion is effected.

While a hub section 25 has been described hereinbefore with a central aperture and a plurality of surrounding elongate apertures for receiving respectively, the pilot and bolts, any other conventional approach may be employed in the hub section 25 for fastening the flexible fan 11 into a driven relationship with the engine 15.

If the flexural groove 53 extends only the major portion of its blade 21 for someimprovement over the prior art, it may extend from the tip end of the fan blade inwardly past the radially outermost tip of the spider arm 23; and it is preferable that it extend at least in this region. On the other hand, it may extend from the innermost end of the fan blade radially outwardly for the major portion of the fan blade, if desired, to afford some help.

The hinge portion 39 provides a controlled line through which the blade will flex during increased load. The formed radius becomes a controlled induced strain area distributed over a large area so as to greatly reduce the fatigue problem encountered in the prior art where the blades were allowed to load or flex through an uncontrolled or concentrated area of stress that fre- This invention provides a hinge portion that does not become tangent to and touch the spider arms at any portion so as to provide full flexure and a full streamlin-.

ing of the flow-inducing portion behind the spider arms for surprisingly low power consumption, elimination of wear at the line of contact between the spider arms and the fan blades, and the other, advantages delineated hereinbefore. In addition, this invention provides a hinge portion that is uniformly arcuate throughout its length, preferably the length of the fan blade 21; instead of a tapered arcuate groove that is deeper at the outer end. The flexural groove of the hinge portion of this invention is not provided for merely stiffening the fan blade, but allowing full flexure without crystallization, yet providing tip stability and eliminating the troublesometip dance of the prior art flexible fans.

Although this invention has been described with a certain degree of particularity, it is understood that the present disclosure is made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of this invention.

What is claimed is:

1. An axial flow flexible fan for use on variable speed engines for pulling a cooling fluid past a cooling coil, particularly at low speeds, responsive to rotation of a rotary part, including a shaft, driven by said engine, the fan being operable to automatically go to low pitch at higher relative engine and air flow speeds for low power consumption when not needed for pulling the cooling fluid past the cooling coil, comprising:

a. a spider that is adapted for being mounted on and rotated with said rotary part; said spider having an axis of rotation and having a plurality of arms ex tending radially outwardly with respect to said axis of rotation; and i b. a plurality of fan blades that are secured respectively to said arms along a line of attachment and adapted for inducing flow of said cooling fluid in a direction parallel to said axis of rotation; said blades having forward sides that are in the direction from which said cooling fluid flows with respect to rearward sides which effect flow of said cooling fluid; each said blade including:

i. a leading edge portion;

ii. an attachment portion overlapping a portion of its respective said arm and affixed thereto so as to rotate in unison therewith; said attachment portion being planar and co-engaging a planar portion of its said arm;

iii. a trailing flow-inducing portion extending away from said arm and attachment portion; said flowinducing portion having a generally arcuate angularly disposed shape for efficiently effecting a flow of a gaseous cooling fluid at low relative speeds of relative movement between said cooling fluid and said fan and having sufficient flexure to streamline into position behind its'rotating said arm at high said relative speeds for low power consumption; and v iv. a hinge portion intermediate said attachment portion and said flow-inducing portion; said hinge portion comprising:

I. a flexural groove extending the major portion of the length of said blade; said flexural groove traversing parallel with said attachment portion and located transversely and trailingly behind said arm for allowing said flow-inducing portion to go into its fully streamlined position without contact with said arm for full streamlining and low power consumption; said flexural groove having at least one arcuate region the cross sectional radius of curvature of which is, at all lines contained therewithin, within a critical range that is large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough not to protrude beyond the dimensions of said arm in the direction of said flexural groove when said flow-inducing portion is streamlined at high speed such that low power is consumed; and II. a tangentially curved region adjacent said flexural groove and leading into said attachment portion; said tangentially curved region having a radius of curvature large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough to become tangent to the plane of said attachment portion; such that said trailing flow-inducing portions of said fan blades have high pitch at low relative speeds for pulling the cooling fluid past the cooling coil at low speeds and automatically go to a lower pitch at higher realative engine and air flow speeds for lower power consumption when not needed for pulling so much cooling fluid past the cooling coil; said hinge portion providing flexure for streamlining without fatigue failure do to crystallization during the life of the fan blade.

2. The flexible fan of claim 1 wherein said arcuate region of said flexural groove has a radius of curvature along any given longitudinal line that is at least 0.20 inch to prevent said line of stress sufficient to cause crystallization and is no more than 0.30 to be small enough not to protrude beyond the dimensions of said arm when said flow-inducing portion is streamlined at high speed.

3. The flexible fan of claim 2 wherein said arcuate region has a cross sectional shape that is a portion of a 10 cir'clewith a radius of curvature of about0.25 inch i 0.05 inch.

- 4. The flexible fan of claim '2 wherein said tangentially curved region is tangent with said attachment portion, blends smoothly with said arcuate region, and has a radius of curvature along any given longitudinal line contained therewithin that is at least 0.10 inch to prevent said line of stress sufficient to cause'crystallization and is no more than 0.20 inch to become tangent to said plane of said attachment portion.

5. The flexible fan of claim 4 wherein said flexural groove extends forwardly with respect to said attachment portion.

6. The flexible fan of claim 4 wherein said flexural groove extends rearwardly with respect to said attachment portion.

7.' The flexible fan of claim 4 wherein a second curved region is disposed transversely adjacent and trailing said arcuate region said second curved region being connected smoothly with'said arcuate region and leading into said flow-inducing portion; said second curved region having a radius of curvature large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough to lead into said flow-inducing portion without any part of said blade protruding beyond the plane of the forwardmost dimension of said arm and into the airstream whereby low power is consumed at full streamlined position of said flow-inducing portion.

8. The flexible fan of claim 7 wherein said second curved region has a radius of curvature along any given longitudinal line contained therewithin that is at least 0.10 inch to prevent said line of stress sufficient to cause crystallization and is no more than 0.20 inch such that said flow-inducing portion does not protrude forwardly of said arm when in its fully flexed, streamlined position.

9. The flexible fan of claim 8 wherein said arcuate region has a cross sectional shape that is a portion of a circle with a radius of curvature of about 0.25 inch 2*: 0.05 inch; and said tangentially curved region and said second curved region have respective cross sectional shapes that are respective portions of a circle curved oppositely to said arcuate region and having respective radii of curvatures of about 5/32 inch i 0.050 inch.

10. The flexible fan of claim 9 wherein said flexural groove extends forwardly with respect to said attachment portion.

11. The flexible fan of claim 9 wherein said flexural groove extends rearwardiy with respect to said attachment portion.

12. The flexible fan of claim 8 wherein said tangentially curved region, said arcuate region of said flexural groove and said second curved region are so dimensioned and connected that the forwardmost part of said flow-inducing portion is rearward of the front plane of said attachment portion by a distance D.

13. The flexible fan of claim 12 wherein D is at least equal to the thickness of said blade and no more than 0.10 inch. Y

14. The flexible fan of claim 1 wherein said flexural grooves extend from the radially interior ends of said fan blades outwardly.

15. The flexible fan of claim 1 wherein the flexural grooves extend the entire length of said fan blades for extraordinary stability and length of service 'with essentially no objectionable tip dance and with low noise.

ill

16. The flexible fan of claim 1 wherein each said fan blade has a plurality of attachment locations extending radially outwardly along said attachment portion to define an attachment line; said leading edge portion is disposed forwardly of said attachment line; and the line of 5 its leading edge intersects a line defined by a planar projection of the curved end of said fan blade at a point that is substantially aligned with said attachment line such that tip dance at high speed is maintained at substantially zero.

17. The flexible fan of claim 16 wherein said spider has substantially planar construction that is substantially perpendicular to its axis of rotation for reduced air drag with low power required for rotation and in-' cludes two halves; and said two halves are disposed one on each side of a planar attachment portion of each said blade; each said arm of each said half, extending radially along the major portion of the length of each said fan blade; each said arm having said attachment line near its trailing edge and having forwardly extending leading edge bracing the radially innermost portion of said leading edge portion of, said fan blade.

18. The flexible vfan of claim 17 wherein the central longitudinal axis of said arcuate region isllocated about 0.75 i 0.20 of an inch transversely trailing said attachment line of said fan blade and transversely trailing said arm.

19. The flexible fan of claim 1 wherein said flexural groove extends forwardly with respect to said attachment portion.

20. The flexible fan of claim 1 wherein said flcxural.

groove extends rearwardly with respect to said attachment portion.

21. The flexible fan of claim 4 wherein said fan blade inch. 

1. An axial flow flexible fan for use on variable speed engines for pulling a cooling fluid past a cooling coil, particularly at low speeds, responsive to rotation of a rotary part, including a shaft, driven by said engine, the fan being operable to automatically go to low pitch at higher relative engine and air flow speeds for low power consumption when not needed for pulling the cooling fluid past the cooling coil, comprising: a. a spider that is adapted for being mounted on and rotated with said rotary part; said spider having an axis of rotation and having a plurality of arms extending radially outwardly with respect to said axis of rotation; and b. a plurality of fan blades that are secured respectively to said arms along a line of attachment and adapted for inducing flow of said cooling fluid in a direction parallel to said axis of rotation; said blades having forward sides that are in the direction from which said cooling fluid flows with respect to rearward sides which effect flow of said cooling fluid; each said blade including: i. a leading edge portion; ii. an attachment portion overlapping a portion of its respective said arm and affixed thereto so as to rotate in unison therewith; said attachment portion being planar and coengaging a planar portion of its said arm; iii. a trailing flow-inducing portion extending away from said arm and attachment portion; said flow-inducing portion having a generally arcuate angularly disposed shape for efficiently effecting a flow of a gaseous cooling fluid at low relative speeds of relative movement between said cooling fluid and said fan and having sufficient flexure to streamline into position behind its rotating said arm at high said relative speeds for low power consumption; and iv. a hinge portion intermediate said attachment portion and said flow-inducing portion; said hinge portion comprising: I. a flexural groove extending the major portion of the length of said blade; said flexural groove traversing parallel with said attachment portion and located transversely and trailingly behind said arm for allowing said flow-inducing portion to go into its fully streamlined position without contact with said arm for full streamlining and low power consumption; said flexural groove having at least one arcuate region the cross sectional radius of curvature of which is, at all lines contained therewithin, within a critical range that is large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough not to protrude beyond the dimensions of said arm in the direction of said flexural groove when said flow-inducing portion is streamlined at high speed such that low power is consumed; and II. a tangentially curved region adjacent said flexural groove and leading into said attachment portion; said tangentially curved region having a radius of curvature large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough to become tangent to the plane of said attachment portion; such that said trailing flow-inducing portions of said fan blades have high pitch at low relative speeds for pulling the cooling fluid past the cooling coil at low speeds and automatically go to a lower pitch at higher realative engine and air flow speeds for lower power consumption when not needed for pulling so much cooling fluid past the cooling coil; said hinge portion providing flexure for streamlining without fatigue failure do to crystallization during the life of the fan blade.
 2. The flexible fan of claim 1 wherein said arcuate region of said flexural groove has a radius of curvature along any given longitudinal line that is at least 0.20 inch to prevent said line of stress sufficient to cause crystallization and is no more than 0.30 to be small enough not to protrude beyond the dimensions of said arm when said flow-inducing portion is streamlined at high speed.
 3. The flexible fan of claim 2 wherein said arcuate region has a cross sectional shape that is a portion of a circle with a radius of curvature of about 0.25 inch + or - 0.05 inch.
 4. The flexible fan of claim 2 wherein said tangentially curved region is tangent with said attachment portion, blends smoothly with said arcuate region, and has a radius of curvature along any given longitudinal line contained therewithin that is at least 0.10 inch to prevent said line of stress sufficient to cause crystallization and is no more than 0.20 inch to become tangent to said plane of said attachment portion.
 5. The flexible fan of claim 4 wherein said flexural groove extends forwardly with respect to said attachment portion.
 6. The flexible fan of claim 4 wherein said flexural groove extends rearwardly with respect to said attachment portion.
 7. The flexible fan of claim 4 wherein a second curved region is disposed transversely adjacent and trailing said arcuate region; said second curved region being connected smoothly with said arcuate region and leading into said flow-inducing portion; said second curved region having a radius of curvature large enough to prevent a line of stress high enough to cause crystallization under flexure and small enough to lead into said flow-inducing portion without any part of said blade protruding beyond the plane of the forwardmost dimension of said arm and into the airstream whereby low power is consumed at full streamlined position of said flow-inducing portion.
 8. The flexible fan of claim 7 wherein said second curved region has a radius of curvature along any given longitudinal line contained therewithin that is at least 0.10 inch to prevent said line of stress sufficient to cause crystallization and is no more than 0.20 inch such that said flow-inducing portion does not protrude forwardly of said arm when in its fully flexed, streamlined position.
 9. The flexible fan of claim 8 wherein said arcuate region has a cross sectional shape that is a portion of a circle with a radius of curvature of about 0.25 inch + or - 0.05 inch; and said tangentially curved region and said second curved region have respective cross sectional shapes that are respective portions of a circle curved oppositely to said arcuate region and having respective radii of curvatures of about 5/32 inch + or - 0.050 inch.
 10. The flexible fan of claim 9 wherein said flexural groove extends forwardly with respect to said attachment portion.
 11. The flexible fan of claim 9 wherein said flexural groove extends rearwardly with respect to said attachment portion.
 12. The flexible fan of claim 8 wherein said tangentially curved region, said arcuate region of said flexural groove and said second curved region are so dimensioned and connected that the forwardmost part of said flow-inducing portion is rearward of the front plane of said attachment portion by a distance D.
 13. The flexible fan of claim 12 wherein D is at least equal to the thickness of said blade and no more than 0.10 inch.
 14. The flExible fan of claim 1 wherein said flexural grooves extend from the radially interior ends of said fan blades outwardly.
 15. The flexible fan of claim 1 wherein the flexural grooves extend the entire length of said fan blades for extraordinary stability and length of service with essentially no objectionable tip dance and with low noise.
 16. The flexible fan of claim 1 wherein each said fan blade has a plurality of attachment locations extending radially outwardly along said attachment portion to define an attachment line; said leading edge portion is disposed forwardly of said attachment line; and the line of its leading edge intersects a line defined by a planar projection of the curved end of said fan blade at a point that is substantially aligned with said attachment line such that tip dance at high speed is maintained at substantially zero.
 17. The flexible fan of claim 16 wherein said spider has substantially planar construction that is substantially perpendicular to its axis of rotation for reduced air drag with low power required for rotation and includes two halves; and said two halves are disposed one on each side of a planar attachment portion of each said blade; each said arm of each said half extending radially along the major portion of the length of each said fan blade; each said arm having said attachment line near its trailing edge and having forwardly extending leading edge bracing the radially innermost portion of said leading edge portion of said fan blade.
 18. The flexible fan of claim 17 wherein the central longitudinal axis of said arcuate region is located about 0.75 + or - 0.20 of an inch transversely trailing said attachment line of said fan blade and transversely trailing said arm.
 19. The flexible fan of claim 1 wherein said flexural groove extends forwardly with respect to said attachment portion.
 20. The flexible fan of claim 1 wherein said flexural groove extends rearwardly with respect to said attachment portion.
 21. The flexible fan of claim 4 wherein said fan blade is of unitary construction, is formed of stainless steel, said flow-inducing portion is formed by bending about a cylinder having a radius of about 0.75 inch and releasing to allow it to flex back into its correct shape; said arcuate region of said flexural groove is formed by bending about a cylinder having a radius of about 0.24 inch. 